It is believed that there are global warming effects that are being caused by the introduction of increased carbon dioxide into the atmosphere. One major source of carbon dioxide emission is the flue gas that is exhausted as a result of a power generation plant's combustion process. Therefore, there have been several efforts by governments and utility companies worldwide, to reduce these emissions.
There are two principal types of power plants that are based on combustion processes; coal combustion and natural gas combustion. Both of these processes produce carbon dioxide as a byproduct when generating power. Efforts have been made to increase the efficiency of the burner, and, therefore, the basic combustion process itself. The intent of these efforts has been to reduce carbon monoxide (the result of imperfect combustion), oxides of nitrogen, and other pollutants. However, since the production of carbon dioxide and water are the basic products of the chemical reaction of combustion, the most efficient technique to minimize the carbon dioxide emission is to capture as much of the carbon dioxide as possible as it is being created by the power plants. In order to truly maximize the efficiency of this technique, existing coal combustion plants, which represent a large portion of the power generation plants worldwide, must also be targeted. The oxy-combustion technique is very interesting, and has significant advantages, since it can be adapted to existing facilities.
Traditional power plants use air as the source of oxidant to combust the fuel (typically coal). Steam is generated by indirect heat exchange with the hot combustion products. The steam is then expanded in turbines to remove useful energy, and, thereby, produce power. The combustion process produces carbon dioxide as a by-product, which is mixed with the residual nitrogen of the combustion air. Due to the high content of nitrogen in the inlet air (78 mol %), the carbon dioxide is diluted in the flue gas. To insure full combustion, the power plants must also run with an excess air ratio that further dilutes the carbon dioxide in the flue gas. The concentration of carbon dioxide in the flue gas of an air combustion plant is typically about 20 mol %.
This dilution of the carbon dioxide increases the size and the power consumption of any carbon dioxide recovery unit. Because of this dilution, it becomes very costly and difficult to recover the carbon dioxide. Therefore, it is desirable to produce flue gas with at least about 90% to 95 mol % carbon dioxide, in order to minimize the abatement cost. The current technology for carbon dioxide recovery from flue gas utilizes amine contact tower to scrub out the carbon dioxide. However, the high amount of heat that is needed to regenerate the amine and extract the carbon dioxide, reduces the amine processes cost effectiveness.
In order to avoid the dilution of carbon dioxide in the nitrogen, the power generation industry is switching to an oxy-combustion process. Instead of utilizing air as an oxidant, high purity oxygen (typically about 95% purity or better) is used in the combustion process. The combustion heat is dissipated in the recycled flue gas concentrated in the carbon dioxide. This technique makes it possible to achieve a flue gas containing between about 75 mol % and 95 mol % carbon dioxide. This is a significant improvement over the previous concentration of about 20 mol %, which is obtained with air combustion. The purity of carbon dioxide in oxy-combustion's flue gas ultimately depends on the amount of air leakage into the system and the purity of oxygen being utilized. The necessary high purity oxygen is supplied by an air separation unit.
In one example of the traditional oxy-combustion process, the carbon dioxide removal process begins as the flue gas exiting the boiler is cooled and sent to an electrostatic precipitator. A portion of the flue gas is further cooled, the moisture is removed, and this portion of the flue gas is recycled to the coal handling section (mill, dryer, etc). Another portion of the flue gas is recycled back to the boiler, and the remaining portion is extracted as flue gas output and is sent to the carbon dioxide purification unit. One example of this type of oxy-combustion is illustrated in FIG. 1.
Since pure oxygen, hence power input and capital cost, is required in the oxy-combustion process to facilitate the capture of carbon dioxide, the whole process, including the oxygen plant and the carbon dioxide capture and purification must be very efficient to minimize the power consumption. Otherwise, the economics of the carbon dioxide recovery will become unattractive to the operator of the power generation plant. In summary, the carbon dioxide capture with oxy-combustion is appealing in terms of pollution abatement, however, in order to achieve it, the capital expenditure and the power input must be minimized to avoid a prohibitive increase in power cost.
As previously mentioned, carbon dioxide purities of 90% or higher (typically 95% or higher) are desirable for many subsequent carbon dioxide abatement techniques (such as deep well injection, deep sea injection or enhanced oil recovery systems). Due to air leakage and the presence of inert gases in the high purity oxygen (nitrogen and argon), in practice the flue gas can be as low as about 75% carbon dioxide. The carbon dioxide concentration must therefore be increased to 90% to 95% in some type of purification process. Common industry specifications typically require that the overall carbon dioxide recovery ratio must be about 90% and even higher than 95% in some cases.
On example, of such a purification system, was described in the Publication of IEA Green House R&D Programme-Oxycombustion Processes for CO2 Capture From Power Plant (Report Number 2005/9, dated July, 2005). This process is illustrated in FIG. 2.
In the process indicated in FIG. 2, the flue gas is washed. Its acid content is removed, it is compressed to a pressure greater than about 30 bar, then it is dried (stream 1). A cryogenic partial condensation process is then utilized to concentrate the carbon dioxide (stream 7 and stream 8).
The carbon dioxide is further compressed to very high pressure (between about 80 bar and about 120 bar) (stream 9). The off-gas leaving the process at 30 bar (stream 10) is generally heated to about 300° C., then it is expanded in a hot gas expander in order to more efficiently recover the potential energy.
In order to heat to 300° C., the gas must be heated first to about 150° C. by exchanging heat with an adiabatic compressor (i.e. the compression heat is not removed by an intercooler, and the exit temperature is allowed to rise to about 200° C.). The gas is then heated to 300° C. by heat exchange with the flue gas from the boiler.
As evidence of these thermal costs, it is noted that an adiabatic compressor (either feed gas or carbon dioxide compressor) consumes more power than the isothermal compressor equipped with intercoolers. Also, the hot gas expander, because of the high expansion ration, (about 30 to 1) and high operating temperature, requires a multiple stage (usually axial type) expander. The skilled artisan will recognize that this type of expander is typically quite expensive. And the heating of the off-gas from about 150° C. to about 300° C. by the flue gas consumes the valuable heat of the boiler, and, therefore, it is possible that steam production will be effected. This will then result in a lower power output from the stream turbines. This reduces the efficiency of the overall process. This also requires a gas-to-gas heat exchanger in the boiler, which, is typically, very expensive. Furthermore, utility companies involved with oxycombustion are also evaluating techniques to minimize the air leakage to further improve the CO2 content of flue gases. This effort also reduces the flowrate of the off-gas stream, such that its recoverable energy becomes smaller, compared with the total power input. Therefore, it becomes less attractive to use less efficient adiabatic compressors to recover the reduced power content of lower off-gas flow.
In another example of the existing art, European patent number 0503910 presents a process scheme, wherein the compressed dry flue gas is treated in 2 distillation columns arranged in series. The first column removes the inert gases (O2, N2 and Argon) and produces a bottom liquid containing CO2, acid gases, and less than 5 ppm O2. This liquid then feeds in the second column, which then yields the pure CO2 overhead liquid and the acid gases bottom liquid. Since these products are in liquid form, this process requires intensive cooling by external refrigeration equipment and additional nitrogen expansion by the oxygen plant. The inert gas extracted from the flue gas is expanded in 3 expanders in series with intermediate reheats to keep the exhaust temperatures of the expanders above the freezing point of CO2.
For the foregoing reasons, a need exists for a more cost effective and efficient method for removing carbon dioxide from the flue gas that is generated by oxy-combustion plants. In particular, a need exists for a method that recovers energy from the expansion of the off-gas stream in a more efficient and cost effective manner.